i got the SK kit on my r32 gtst with subframe bushes.
i get lots of axel tramp....its definitely that jumpy motion.
Its so violent that i have to back of the throttle because it feels like something will break.
I can confirm that have the whiteline bushes inthe max traction postion made it worse. The all round setup seems to help but only slightly.
I've also run, nexen cheap tyres, Re55's, 595RS, 595SS. The issue is only slightly different with each of these tyres.
Car is at ~350mm
my 2 cents......i will look into the diff idea.